Refrigerators designed for use in personal residences are of two basic types. One type does not provide any means whatsoever for effecting defrosting of the evaporator that is placed in a compartment for effecting freezing of foods. It is thus necessary at periodic time intervals to perform manual operations to effect removal of frost and ice that accumulate over a period of time on the evaporator surfaces. Refrigeration apparatus of this type frequently utilizes an evaporator embodying a construction that is designated as a plate-form construction that is utilized in substantial part because of its economy of manufacture. A plate-form evaporator also has a structural advantage in that it can also perform the auxiliary function of supporting the food products.
Defrosting of a refrigerator, specifically the freezer evaporator, is a time-consuming task and generally requires that the refrigerator be placed in an inoperative condition for a period of time. It also frequently requires that the food products be removed from the freezer compartment in order that appropriate measures may be taken to apply the necessary quantity of heat to the evaporator surfaces to effect the defrosting and, in particular, to effect sufficient melting of ice accumulations so that the ice may be removed. Defrosting techniques currently employed often involve the placement of containers of heated water in the freezer compartment and either adjacent to or supported on the evaporator. This technique invariably results in the further complication because melted frost and ice generate water that must somehow be picked up and manually removed. Particles, or chunks of ice, are also invariably produced and they must be removed. Generally, this manual defrosting procedure creates a significant cleanup problem.
To alleviate this defrosting problem, refrigeration apparatus for domestic use has been designed to incorporate structure and control systems to effect automatic defrosting. Such refrigeration systems with automatic defrosting features usually incorporate an evaporator which is of a construction that is generally designated as a "fin coil". This fin coil is placed in the freezer compartment and is functionally connected with other components of the refrigeration system such as the compressor, a condensor that is mounted exteriorly of the freezer compartment and the necessary control devices such as an expansion valve. Defrosting is effected by application of heat to the fin coil and this is accomplished at periodic intervals normally controlled by electrical circuit timers. While the current technology of automatic defrosting refrigeration systems do accomplish the intended objectives, they are not efficient from a cost standpoint in operation and, furthermore, they require incorporation of components that substantially increase the basic cost of the refrigeration apparatus. Heat for effecting defrosting is normally provided by means of an electric heating element. This heating element must be placed in close proximity to the fin coil evaporator in the freezer compartment and this factor further adds to the increased costs of automatic defrost freezers in that it significantly complicates the design and configuration of the evaporator as well as restricting their physical location in the freezer compartment.
These current technology automatic defrosting systems also are costly to operate. Sufficient heat must be applied to effect melting of the ice and/or the frost that is formed on the fins of the evaporator. To complete defrosting, it is necessary to apply this heat over a fairly prolonged period of time in order to assure that there will be sufficient heat transfer to effect the melting. Ice is an insulator and this further adds to the complications of defrosting as the heat must be applied for a sufficient time to melt the ice until the heat actually reaches the evaporator surfaces. With ice being a thermal insulator, it is also undesirable to permit large accumulations of ice to form on the evaporator as the ice will then also seriously erode the cooling or freezing efficiency of the system. In a defrost mode of operation, the electrical defrost heater must produce heat at a rate which is greater than the cooling capacity of the freezer system and often this heat generation must be at a rate which is three to four times the cooling capacity rate. Further adding to the inefficiency of the automatic defrosting system is the fact that the electric heater necessarily generates space heat that is dissipated into the freezer area. Once the defrosting cycle has been completed, it then becomes necessary for the refrigeration system to then work in the freezing cycle for an extended period of time to effect removal of this added heat. The cost of operating the freezer provided with automatic defrosting is relatively greater than with a freezer that does not have the automatic defrost feature. The comparative relative cost of operation may well be in the order of the automatic defrost system consuming two to three times the energy that is otherwise required by a refrigerator which is not provided with an automatic defrost system.
A further disadvantage of the current technology systems of automatic defrost refrigeration apparatus is that such systems cannot be applied or incorporated with most commonly designed freezers such as those with the plate-type evaporators. These plate-type evaporators are of a design which covers a relatively large physical area, and thus, they are not readily adapted to having means for applying heat to effect defrosting such as is the case with the fin coil type evaporators. In addition to the lower cost refrigerators which are provided with such plate-type evaporators, most common upright or chest-type freezers adapted for utilization in residences are also of the type having such plate-form evaporators. Consequently, these freezers which essentially comprise the types that are found in residences, and many commercial installations, are also not adapted to automatic defrosting and must be defrosted by manual techniques.